Social spiders are an arachnophobe’s nightmare. While the vast majority of spiders work alone, the odd few live communally and cooperate to hunt and feed. Their numbers, along with the massive webs that they all have a spinneret in creating, allow them to tackle prey far larger than themselves. The aftermath of a kill opens up new conflicts for the spiders that other cooperating hunters like lions or wolves don’t share. They don’t divide up the carcass to eat separately, for like all spiders, they digest their prey outside their bodies.

All the colony members spit their digestive enzymes into the corpse and suck up the liquefied remains. But producing these enzymes costs energy, and since all the spiders are using the same opaque spittoon, there’s no way for an individual to tell how much its teammates are coughing up. It’s a system that favours cheats, who take their share of the communal resources but contribute very little toward creating them. This conflict isn’t just theoretical – experiments have shown that groups of social spiders feed far less efficiently than individuals do.

In the face of this temptation to cheat, how do the spiders manage to eat anything at all? This is a perennial question faced by biologists who seek to understand the evolution of cooperation: why work together, when cheating often yields greater rewards? There are many possible answers and in the social spiders, Jutta Schneider and Trine Bilde saw a chance to test one of the most pervasive – the theory of kin selection.

The theory holds that individuals can find extra value in helping their relatives because there’s a good chance that their bodies house many of the same genes. By ensuring their survival, animals can increase the chances that their own genes are passed to the next generation, no matter how indirectly. The role of kin selection among the many other theories of cooperation is still unclear, and particularly whether it could help to foster the initial stages of cooperation, paving the way for other processes to take hold.

Schneider and Bilde tested the role of kin selection in Stegodyphus lineatus, a species of spider that isn’t quite fully social yet. Unlike true social spiders that live permanently in groups, Stegodyphus only forms alliances as youngsters – it’s a ‘subsocial’ spider. The species breeds only once in its life for the mother offers her own bodily fluids as a first snack for her newly hatched young, who promptly drink her dry. For the next couple of months, the young spiders stay in the next together and hunt as a group.

Sorta-social

Schneider and Bilde collected female spiders and their egg sacs from the Greek island of Karpethos. After hatching, the spiderlings were removed and swapped around. Some siblings were kept as a group and returned to their mothers as normal. Others were mixed and matched before being assigned a parent, and some of these spiders were given a chance to get to know one another before they were returned.

Over the next 8 weeks of feeding and growing, the siblings put on weight at a significantly quicker rate than the groups of unrelated spiders and ended up weighing more on average. Their siblings’ boosted growth was down to a more efficient feeding style, and they managed to extract more mass from their prey than the unrelated groups did. And because of these larger meals, fewer spiders in the related groups had died by the 8-week mark.

You might argue that related spiders are simply more familiar with each other, but Schneider and Bilde’s experiment allowed them to separate the effects of acquaintance from genetic relatedness. In the two unrelated groups, the spiders that were introduced to one another beforehand didn’t cooperate to any greater extent than the fresh strangers.

The fact that cooperating relatives fared better than unrelated teams provides strong support for the idea that kin selection helps to mitigate the negative effects of competition over a common resource. The advantages of this nepotism could provide a foothold for developing the type of fully social lifestyle seen in the tremendously successful social insects.

Of course, the edge displayed by the related groups suggests that Stegodyphus has ways of telling family members from strangers. How they do this is a mystery, but other studies have also suggested that they can. If they lack for prey, the spiders will turn on each other but they are much more likely to cannibalise unrelated individuals than relatives.

Comments (6)

Completely and disgustingly awesome. Great write up and pix – I think I sent this article to about 20 people – ten of which are already emailing me back with something along the same lines. Now, if only we had restaurants that offered similar fare…

Jonathan – How dare you write digusting and spiders in the same sentence? Oh, wait, you didn’t! My bad.
But they’re so cute. Don’t you just love their little black eyes and gently waving palps? And they’re so cuddly and comic. Plus they generously allowed their invention to lend its name to the world wide web.

One of the most interesting observations as i am a spider enthusiast was the semi social rather colonial behaviour of cyrtophora citricola spider in my garden in the spring-summer-autumn of 2010. At first during the winter (which was rather not very cold) was a single nest in a bush near the fence of my garden. But when the spring came a colonial explosion happened. Started late March with 3 nests of cyrtophoras one of top of the other and when May reached the nests were about 20 on top of each other covering half a tree and a fence. New nests in early May were created by new cyrtophoras close to the main site enxtending all directions. Finally in June all the nests covered an area about 3 metres in height and 2 metres in span. Nests were extending out of the fence at the fence covering a lemon tree upwards to bushes 2 metres above. The site was clearly visible oposite the sun from 70 metres away and was huge. Close to the site were similar colonies smaller in size. The neighborhood was fuly emerged with the same spider. You could see them on pine trees having a nest more that 8 metres from the ground (very rare) on bushes and trees like phoenix (not phoenix though) The most exciting was near the afternoon when they caught prey at the site . A single vibration made many females some of the nests to break into the nest where it was caught and the result was epic battles for the prey. During that time more than 15 large females and about 30 smaller females completely emerged half the tree and was the dominate predator for all mosqitoes, flies, bees and insects. I could see the site above from my balcony and observe and when a singe vibration of a bee was observed then citricolas were behaving like maniacs to reach first. In the summer their behaviour was rather calmer as they attended their eggs (looked like peas one stick to the other) if they caught prey the battle was not so big but if a stranger tried to break in they were more hostile obvioulsly to defend the eggs. Then in july spiderlings gave birth. The site probably contained at times more than 500 or even 1000 spiders of all sizes. Each younger made his own nest around the nest of their mother (but then the number got smaller probable predators other spiders even canibalism between them) The peak of all births was in late July and then the site begun to fade late September. Around the garden of my house (north subburbs of Athens- Greece) and out of its borders there were more than 50 nests and apart from the colony discribed were other 5 more colonies at trees among the street and other empty spaces near my house. This year i haven’t seen any cytophoras apart form 2 little ones in my garden last week. Probably the reason for such an explosive development of that spider was last’s year calm winter but for sure such a site is very rare as where i live cyrtophoras as a spider is rare and Athens is a heavy populated area extending more and more to the north. In general it was very exciting to observe this site as every day it was expanding especially during spring and to see “the battles” for the preys. And the preys were not caught by me it was fully naturally observed…